(low priority) default values or errors for trigger / goal condition / check status / step

[AndrewBKang]

when creating an agent a check_status must be supplied even if it will be empty. (this may be a feature not a bug) but perhaps it might be valuable to have a default empty block. I do not know if check_status is frequently needed for agents, I suppose it would be a matter of frequency of its requirement.

For trigger/goal condition/ step (they seem more required) it may be helpful to get some kind of not implemented messaging.